Methods for Producing Triol Ethers by Reactive Distillation

ABSTRACT

This invention relates to methods for preparing alkyl ethers from glycerin by reactive distillation. For example, the present invention provides a method for the preparation of mono-, di- and tri-ethers of glycerin, either in the presence or absence of fatty acids, contaminants by reactive distillation using solid catalysts. Specific desirable final products according to the reactive distillation method provided herein include glycerin ethers and fatty acid esters of C 4 -C 5  alcohols, such as for example, isobutanol, tert-butanol, and isoamyl alcohol.

This application claims priority under 35 U.S.C. 119(e) to U.S. provisional application 60/894,724, filed Mar. 14, 2007, U.S. provisional application 60/894,726, filed Mar. 14, 2007, and U.S. provisional application 60/894,730, filed Mar. 14, 2007. The contents of each of the above-listed applications are incorporated by reference.

FIELD OF THE INVENTION

This invention relates to methods for preparing alkyl ethers from glycerin by reactive distillation.

BACKGROUND

Glycerin (propane-1,2,3-triol, also known as glycerol), is a product of the splitting of triglycerides from fats as the triglycerides are separated into component fatty acids and glycerin. During transesterification of triglycerides (typically conducted in the presence of basic catalysts), large amounts of crude glycerin are produced. For example, in the transesterification of triglyceride oil, such as for example, that obtained from soybeans, with methanol, approximately 20% of crude glycerin is generated for which applications must be found. Purification of glycerin obtained from transesterification for commercial application is difficult and expensive, even to obtain poor quality product of questionable value. One major constituent of crude glycerin is fatty acid from the original triglyceride oil.

Currently, the relatively high price of biodiesel compared to diesel oils derived from petroleum is one obstacle to the complete commercial acceptance of biodiesel as an alternative fuel source. Additionally, use of biodiesel fuels is frequently limited in practice, due in part to inferior physical properties at low temperatures. For example, the cloud point of soya biodiesel (the lowest temperature at which a fluid can remain as a fluid without becoming turbid or beginning to crystallize) is near zero degrees centigrade (0° C.), whereas the cloud point of petroleum-derived diesels is typically around −16° C. Similarly, freezing points for biodiesel oils are around −2° C., compared with −27° C. for petroleum-derived diesel oils. These inferior physical properties of the biodiesel compared to conventional petroleum based diesels cause problems at low temperatures. A viable new use for crude glycerin byproduct improves the economics of the transesterification of triglycerides. It further benefits the biodiesel producer if the product formed from the glycerin can be used to improve physical properties of biodiesel.

One viable candidate for such glycerin based products includes alkyl ethers of glycerin. Production of glycerin ethers from an alcohol such as isobutanol yields mono-, di-, and tri- (tertiary) butyl ethers of glycerol.

A further advantage in the production of alkyl ethers of glycerin is that the same class of acid catalysts such as for example, Amberlyst resins and the like catalyze both the etherification of glycerin and the esterification of fatty acids that will most likely contaminate glycerin produced during Biodiesel production.

Because of reaction equilibrium constraints, the etherification of glycerin readily achieves high conversions when conducted simultaneously with vapor liquid equilibrium stage operations. Those skilled in the art refer to the combination of reaction and vapor liquid equilibrium stage operations as “reactive distillation”.

Therefore, procedures to transform glycerin by itself, or in admixture with the fatty acids that normally contaminate it, into compounds that can be mixed with biodiesel to improve the biodiesel properties have been investigated. Specifically, the production of compounds to improve biodiesel properties at low temperature and to improve combustion is an objective of great technical and commercial value.

SUMMARY

The present invention provides a method for the preparation of mono-, di- and tri-ethers of glycerin, either in the presence or absence of fatty acids, contaminants by reactive distillation using solid catalysts. Specific desirable final products according to the reactive distillation method provided herein include glycerin ethers and fatty acid esters of C₄-C₅ alcohols, such as for example, isobutanol, tert-butanol, and isoamyl alcohol.

The process includes continuously introducing an alcohol vapor feedstream and a glycerol feedstream to a distillation column. Preferably, the alcohol feedstream is introduced to the bottom of the distillation column as a vapor and often to the top as a liquid, while the glycerin feedstream, which may include fatty acids, is introduced to the top of the distillation column. The alcohol and glycerin, as well as any fatty acids, are catalytically reacted in the combination reaction/distillation zone of the column. The vapor liquid equilibrium stages ensure that water produced by the production of ethers and esters is removed from the reaction phase as it is formed, thereby favoring a higher conversion than permitted by the reaction equilibrium when the water is allowed to remain in the reaction phase.

The reaction column is operated such that water and excess alcohol exit as a vapor from the top of the column. Water may be separated from the excess alcohol and the alcohol may be recycled to the reaction column. Product mono-, di- and tri-triol ethers along with any fatty acid esters formed exit the column from the bottom as a liquid.

In one embodiment, the alcohol is selected from isobutanol, tert-butanol, and isoamyl alcohol. In one embodiment, the reaction zone includes a solid ion exchange catalyst, wherein the catalyst includes SO₃H and CO₂H reactive groups.

In a preferred embodiment, isobutanol and/or tert-butanol is supplied to the bottom of a distillation column in vapor form and liquid glycerol (with or without fatty acid contamination) is supplied to the top of the distillation column in liquid form. It may also be desirable under certain conditions to supply liquid butanols to the top of the column along with the vapor to the bottom of the column. The isobutanol and/or tert-butanol proceed counter currently with the glycerin and fatty acids and through the vapor liquid equilibrium stages simultaneously reacting to their respective ethers and esters. The equilibrium stages are designed to hold a solid catalyst, preferably an ion exchange resin having either —SO₃H or —CO₂H functional groups present, in such a way that the catalyst is exposed to liquid containing alcohol and either glycerin or fatty acid or both. Water is removed from the reaction phase as it is created. The tert-butanol, isobutanol and water are subsequently separated using any of a wide range of separation technologies such that the alcohols may be recycled to the bottom of the distillation column as a vapor and to the top of the column as a liquid, if desired. Mono-, di- and tri-tert-butyl ethers of glycerin, and the tert-butyl esters of fatty acids if present in the feed glycerin, are collected from the bottom of the distillation column.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment for the production of triol ethers by reactive distillation.

FIG. 2 shows an embodiment for the production of triol ethers by reactive distillation with alcohol recycle.

FIG. 3 shows an embodiment for the production of biodiesel fuel containing a triol ether additive.

FIG. 4 shows an embodiment for the production of triol ethers by reactive distillation with pre-etherification/esterification and alcohol recycle.

DETAILED DESCRIPTION

According to the present invention and with reference to the accompanying drawings, a continuous process for the production of glycerol ethers by reactive distillation of glycerin, potential fatty acid contaminants, and alkanols with a heterogeneous catalyst is provided. The catalyzed reaction of glycerin and alkanol occurs in a reaction column equipped with vapor liquid equilibrium affecting devices. The glycerin ether and fatty acid ester products of the reaction find use as additives that can be used to improve cloud-point, viscosity, pour-point, and cold flow plugging point of biodiesel.

Etherification by Reactive Distillation

As shown in FIG. 1, glycerol (1,2,3-propane triol) 1 is fed via line 2 to the upper portion of and optionally via line 18 to the upper portion of reaction column 5. Similarly, alkyl alcohol 3 is introduced via line 4 to the lower portion of the column 5. Preferably, the glycerol 1 is introduced above the reaction zone 6, and the alcohol 3 is introduced below reaction zone 6. The alcohol is present as a vapor and flows counter-current to the liquid glycerol, which is preferably present in the reaction as a liquid.

As noted herein, any suitable C₁-C₆ straight or branched alcohol may be used, most preferably tert-butanol, isobutanol or mixtures thereof.

Reaction of the alcohol and glycerin produces a mono-ether and water. Subsequent reaction of the mono-ether produces the di-ether and water, and further reaction of the di-ether produces the tri-ether and water.

The column is preferably configured for reactive distillation using a solid catalyst. Such columns employ one or more vapor liquid equilibrium affecting devices that serve to hold the catalyst. As used herein, a vapor liquid equilibrium stage can also be described as a tray or plate. General commercial examples of stages, trays, and plates include bubble cap trays, valve trays, sieve trays, random packing, and structured packing. Regardless of the specific design employed by a given manufacturer, the objectives are to affect vapor liquid equilibrium in a stage-wise fashion and to hold solid catalyst. An exemplary column and several vapor liquid equilibrium stage/catalyst supporting means suitable for use herein are described in U.S. Pat. No. 5,536,856 (Harrison, et al.). A specific, long term commercial product known as Katapak consistent with the some of the designs of the vapor equilibrium stages described in U.S. Pat. No. 5,536,856 is described in U.S. Pat. No. 5,831,120 (Watson et al).

Generally, of the two or three reaction components, the reactant with the lower boiling point is introduced at the bottom of the distillation column and is present in the reaction as a gas, while the reactant with the higher boiling point is introduced at the top of the distillation column and is present as a liquid. The alcohol vapor serves as a stripping vapor, aiding in the removal of water from the reaction vessel, as the majority of the water is exits the distillation column out the top of the vessel with the alcohol effluent.

Reaction zone 6 includes a solid catalyst for the etherification of glycerol. A variety of solid catalysts may be used. Preferably, the catalyst is an ion exchange resin which includes sulfonic acid (—S(O)₂OH) or carboxylic acid (—C(O)OH) reactive groups, or a mixture thereof. Suitable arrangement is made for holding the catalyst in the region where vapor liquid equilibrium is taking place. A synthetic zeolite or other type of mixed or singular oxide ceramic material with sufficient acidity could also be employed. In columns employing multiple incidents of catalyst zones, several different catalysts, or multiple different concentrations of catalyst, may be employed.

In the reactive distillation process according to Harrison (as described in U.S. Pat. No. 5,536,856), the distillation column includes a plurality of vapor liquid equilibrium stage devices that also hold catalyst called “trays”, the number of which may be determined according to the desired volume of the reactor, the boiling points of the reactants, and the desired product (i.e., the mono-, di- or tri-ether). In the reactive distillation process according to Watson (as described in U.S. Pat. No. 5,831,120), the equilibrium stages are called “packing” and may include porous catalyst supports and flow channels for affecting vapor liquid equilibrium and contacting the liquid phase with the catalyst.

The alcohol, having the lower boiling point, exits the distillation column overhead via line 7. Because of the vapor liquid equilibrium stage action of the column and its stages, excess alcohol and water from the etherification reaction exits the column as a vapor from the top. The water/alcohol mixture exiting via line 7 may be separated by known separation processes 8, such as for example, by distillation, and the alcohol may preferably be recycled to the distillation column via lines 11 and 18. Waste water exits the water/alcohol separation process via line 10.

The glycerin ether and fatty acid ester products exit distillation column 5 as a liquid via line 9. The product stream may include mono-, di- and tri-triol ethers, and may also include unreacted glycerol as well as esters of any fatty acids introduced with the glycerin feed.

In order to generate the vapor phase necessary for the vapor liquid equilibrium action of the column, a reboiler (not shown) may be employed. Alternatively, the alcohol fed to the bottom is vaporized by external means.

As shown in FIG. 2, product stream 9, which may include mono-, di- and tri-ethers of glycerol as well as esters of fatty acids may be introduced to a separation process 12. Separator 12 includes means known in the art, such as for example a distillation column, and may preferably be used to produce a product stream 14 rich in di- and tri-ethers of Triol and a recycle stream 13 rich in mono-ethers of Triol and unreacted Triol. Recycle stream 13 may then be combined with feed stream 2, or optionally be introduced separately, to distillation column 5.

As shown in FIG. 3, product stream 14, preferably rich in di- and tri-ethers of Triol, may be added to a mixing process to which biodiesel (preferably fatty acids of methyl esters) is added via line 16. Depending on the biodiesel feedstock, the amount of Triol ether may be adjusted. Preferably, the resulting biodiesel-additive product 17 includes at least 2%, at least 3%, at least 4%, at least 5%, at least 6%, at least 7%, at least 8%, at least 9% or at least 10% by weight Triol ether additive. Any fatty acid esters formed as a result of the presence of fatty acids in the glycerin will have desirable low temperature properties that will compliment the effect of the glycerin ethers when added in combination.

Optionally, the triol ether fuel additive can be directed added to a biodiesel or petroleum based diesel fuel, without further reaction or purification (not shown). Due to the use of an ion exchange resin or structured packing, residual catalyst typically does not have to be removed from the product material. Additionally, prior to addition of the additive, it may be optionally dried by known means, such as for example, by passing through a drying agent (e.g., calcium sulfate).

As shown in FIG. 4, at times it is preferable to perform a portion of the etherification and esterification in a fixed bed reactor containing a charge of catalyst upstream of the reaction column. This helps ensure high conversion, while at the same time reducing the number of stages and amount of catalyst required in the reaction column. As shown in FIG. 4, glycerin stream 2, with or without fatty acids, can be mixed with recovered alcohol stream 24 and introduced to pre-reactor 19. Prereactor 19 can be charged with a catalyst similar to that used in the reaction column. Following the reaction in pre-reaction 19, wherein at least a portion of the glycerin has been etherified and some of the fatty acids have been esterified, the stream 20 is supplied to flash drum 21. The flash drum 21 separates the reaction mixture into a liquid stream 22 and a vapor stream 23, each of which can be fed to the top of the reaction column 5.

Glycerin

As shown in FIG. 4, at times it is preferable to perform a portion of the etherification and esterification in a fixed bed reactor containing a charge of catalyst upstream of the reaction column. This helps ensure high conversion, while at the same time reducing the number of stages and amount of catalyst required in the reaction column. As shown in FIG. 4, glycerin with or without fatty acids are mixed with recovered alcohol from stream 14 and sent to pre-reactor 2. Prereactor 2 is charged with a catalyst similar to that used in the reaction column. Following the reaction in pre-reaction 2, some of the glycerin has been etherified and some of the fatty acids have been esterified. The product of prereactor 2 is sent by stream 3 to a flash drum, 4, for separation of liquid and vapor streams with both being fed to the top of the reaction column, the vapor stream being fed higher.

As shown in FIG. 4, at times it is preferable to perform a portion of the etherification and esterification in a fixed bed reactor containing a charge of catalyst upstream of the reaction column. This helps ensure high conversion, while at the same time reducing the number of stages and amount of catalyst required in the reaction column. As shown in FIG. 4, glycerin with or without fatty acids are mixed with recovered alcohol from stream 14 and sent to pre-reactor 2. Prereactor 2 is charged with a catalyst similar to that used in the reaction column. Following the reaction in pre-reaction 2, some of the glycerin has been etherified and some of the fatty acids have been esterified. The product of prereactor 2 is sent by stream 3 to a flash drum, 4, for separation of liquid and vapor streams with both being fed to the top of the reaction column, the vapor stream being fed higher.

Catalyst

The solid catalysts, capable of catalyzing both the desired etherification and esterification suitable for use in the invention, range from acidic zeolites and other silicas, alumina, and titanias, to granular ion exchange resin containing sulfonyl acid (—SO₃H) and/or carboxylic acid (—COOH) groups. Macroreticular resins of this type are preferred. Examples of suitable resins are those sold under the trade marks “Amberlyst”, “Dowex”, “Dow” and “Purolite” such as AMBERLYST 13, AMBERLYST 66, DOW C351, and PUROLITE C150. The same catalyst can be employed at multiple stages or different catalysts can be used at different stages.

Preferably, the catalyst employed is stable at the temperatures at which the reaction is run. For example, if any of methanol, ethanol, n-propanol, isopropanol, n-butanol or isobutanol are used as the alcohol, then the catalyst (as an ion exchange resin) must be able to be operate at temperatures between 120° C. and 140° C.; preferably having only a moderate activity loss at this temperature range. When alcohols having higher boiling points are employed, the catalyst similarly must be able to operate, and must have only a moderate activity loss, at higher temperatures which correspond to the boiling point of the alcohol being used.

When the distillation column includes trays, the charge of solid particulate or granular etherification catalyst on an equilibrium stage should typically be sufficient to provide a catalyst:liquid ratio on that tray corresponding to a resin concentration of at least 0.2% w/v, for example a resin concentration in the range of from about 2% w/v to about 20% w/v, preferably 5% w/v to 10% w/v, calculated as dry resin. Sufficient catalyst should be present to enable equilibrium or near equilibrium conditions to be established on the tray or in the packing within the selected residence time at the relevant operating conditions. However, the amount of catalyst should be maintained such the upflowing vapor entering the tray from below can sufficiently agitate the catalyst on the tray. For a typical resin catalyst a resin concentration in the range of from about 2% v/v to about 20% v/v, preferably 5% v/v to 10% v/v may be used.

In another embodiment, the catalyst may be a fixed-bed catalyst. In this case, the reaction column may be operated as a trickle column of which about 30 to 60 vol %, preferably approximately 50 vol % are utilized by the stripping gas as free gas space, whereas 30 to 50 vol %, preferably approximately 40 vol % of the column is occupied by solid substance, i.e. the fixed-bed catalyst. The remaining reaction space, preferably approximately 10 vol % or less, may be occupied by the trickling liquid.

The residence time of the liquid phase in the distillation column can be adjusted by the stripping gas velocity. With higher gas velocities, the residence time of the liquid phase is typically high. Generally, the stripping gas throughput can be adjusted over a wide range without having an adverse effect on the course of process.

While many acid catalysts suitable for performing etherifications can be used, in an effective amount and an effective concentration, solid catalysts having acidic functional groups are preferred. Solid catalysts are preferred mainly because of the minimization of purification steps required in processing the product stream. Examples of suitable acids may include, but are not limited to, hydrochloric acid, hydrobromic acid, sulfuric acid, and nitric acid. Preferably, the acid catalyst is a strong acid, more preferably sulfuric acid.

Alcohols

Alcohols suitable for use in the present etherification reaction can include any C₁₋₁₀ straight, branched, or cyclic alcohols. Preferably, the alcohol is a C₄ or C₅ alcohol, such as for example, tert-butanol, isobutanol, and/or isoamyl. Anhydrous alcohols are preferred, although because of the use of a reactive distillation column, any water which is present in the reaction mixture is typically removed by the stripping action of the alcohol vapor. Alcohol will typically be employed in excess to that required stoichiometrically.

Modifications and variations of the present invention relating to a fuel additive composition and an alternative fuel derived from the composition are encompassed in the foregoing detailed description of the invention. Such modifications and variations are intended to come within the scope of the appended claims. Similarly, the drawings are diagrammatic and additional equipment, such as for example, reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. 

1. A process for the production of glycol ethers by reactive distillation, comprising: continuously introducing an alcohol vapor feedstream to a distillation column; continuously introducing a glycol feedstream to the distillation column; catalytically reacting the alcohol and glycol feedstreams in a reaction zone within the distillation column to form; stripping water from the reaction zone with the alcohol vapor; separating the water from the alcohol vapor and recycling the alcohol to the bottom of the distillation column; collecting the mono-, di- and tri-glycol ether products.
 2. The process of claim 1 wherein the alcohol is selected from isobutanol, isoamyl alcohol and tert-butanol.
 3. The process of claim 1 wherein the reaction zone includes a solid ion exchange catalyst, said catalyst including SO₃H and CO₂H reactive groups.
 4. The process of claim 1 wherein the reaction zone includes trays.
 5. The process of claim 1 wherein the reaction zone includes structured packing.
 6. The process of claim 1 wherein the alcohol is introduced to the bottom of the distillation column.
 7. The process of claim 1 wherein the glycerin is introduced to the top of the distillation column.
 8. A process for the preparation of tert-butyl triol-ethers, comprising: continuously introducing tert-butanol to the bottom of a reactive distillation column; continuously introducing glycerin to the top of said distillation column; heating the alcohol to form a stripping vapor; reacting said glycerin and said stripping vapor in a reaction zone, said reaction zone located between the top and bottom of the distillation column; collecting said stripping vapor from the top of the distillation column; recycling said stripping vapor to the distillation column; and collecting a triol-ether product from the bottom of said distillation column. 